The Islamic militant group ISIS cut off the head of an American journalist in a video made public Tuesday — following through on a threat it made when it beheaded another American two weeks ago and vowed to keep killing unless the United States stopped airstrikes.

The journalist, Steven Joel Sotloff, was beheaded in a video made public by SITE Intel Group, which monitors jihadist organizations. The video surfaced days after Sotloff’s mother pleaded with the leader of ISIS for mercy.

ISIS beheaded another American journalist, James Foley, in a video released Aug. 19 and threatened to kill Sotloff unless President Barack Obama ended the airstrikes, which have pummeled ISIS targets in Iraq for four weeks.

Sotloff, 31, wrote about conflict in the Middle East for Time magazine and other publications. He grew up in Miami and attended the University of Central Florida in the early 2000s. He disappeared in August 2013 in Syria.

A family spokesman told NBC News that Sotloff’s relatives were aware of the video, and that while they were waiting for it to be verified, they were grieving.

The video, titled “A Second Message to America,” opens with a clip of President Barack Obama speaking after ISIS beheaded Foley last month. Obama vowed in those remarks to be relentless when Americans are harmed.

The video then shows Sotloff, wearing an orange jump suit and kneeling, in a sparse desert landscape, next to a black-clad ISIS fighter — a replica of the conditions in the video in which Foley was beheaded.

Speaking to the camera, Sotloff faults Obama for U.S. intervention in the Middle East. The ISIS fighter then blames Obama for refusing to heed ISIS’s warnings and end military strikes. The video threatens the life of a British captive, David Cawthorne Haines.

“It is an absolutely disgusting and despicable act,” Prime Minister David Cameron of Britain said, according to the Press Association news agency.

Haines worked for a string of aid organizations, NBC News learned.

The White House press secretary, Josh Earnest, said that he could not confirm the authenticity of the Sotloff video. He said that the United States had devoted significant time and resources to rescuing Sotloff. The CIA and State Department also said that they could not confirm authenticity.

The Committee to Protect Journalists said it condemned the “pure barbarism” in the strongest possible terms.

Sotloff, like Foley, “went to Syria to tell a story,” said Joel Simon, the executive director of CPJ. “They were civilians, not representatives of any government. Their murders are war crimes and those who committed them must be brought to justice swiftly.”